A quartz crystal plate to be used for a conventional ordinary AT cut quartz crystal resonator is flat, and hence the highest fundamental frequency obtainable for practical use is around 40 MHz from present manufacturing techniques and mechanical strength. On this account there has been widely employed what is called overtone oscillation means which extracts a higher order harmonic mode vibration of an AT cut quartz crystal resonator to obtain a frequency which is an odd multiple of the fundamental resonance frequency, but its oscillation circuit requires for an LC tuning circuit including a coil and hence is not suitable for fabrication as an IC, besides the overtone oscillation circuit may sometimes be difficult to maintain at a stable oscillation because a resonator to be included in the overtone oscillator has a large capacitance ratio and consequently a high impedance level. On the other hand, a surface acoustic wave (SAW) resonator, whose resonant frequency is determined by the pitch of electrode fingers of an interdigital transducer, is able to resonate at a maximum of 1 GHz or so due to the progress in photolithography, but a piezoelectric substrate when used as the surface acoustic wave resonator is remarkably inferior to the AT cut quartz crystal in terms of temperature-frequency characteristic. To solve this problem, there has been proposed and is now under study an ultrathin piezoelectric resonator which has a cavity provided, by etching or mechanical grinding, in one side or in both sides of a block of AT cut quartz crystal to form an ultrathin vibratory portion surrounded with a support frame so that the fundamental resonant frequency may extend from ten megahertz to hundreds of megahertz while retaining the mechanical strength of the vibratory portion.
By fabricating a multimode filter element using such an ultrathin piezoelectric substrate as above, a filter with a center frequency in the range from about ten to about 100 megahertz could be easily obtained without utilizing the overtone techniques.
Incidentally, as regards a conventional multimode piezoelectric filter element, it is said to be preferable that when the center frequency of the filter is 10 MHz, the thickness of an electrode to be evaporated onto the piezoelectric substrate should be about 3000 .ANG. so there is entrapping or confinement of the vibration energy at an appropriate value and an ohmic loss which is sufficiently low. The mass of electrodes is one of the parameters in the analysis of the confinement of vibration energy, and the equivalent thickness of electrode films is defined as the sum of the thicknesses of the select films formed on both sides of the quartz crystal substrate to calculate the mass of electrodes. Here, this definition of the thickness of electrode films is employed.
On the other hand, in the case where a filter element which has a center frequency of 100 MHz and is similar to the above-mentioned filter element in the vibration energy entrapping characteristic and other basic characteristics is manufactured using an ultrathin AT cut quartz crystal plate, the electrode should be vapor-deposited to a thickness of 300 .ANG., i.e. 1/10 of the above-said value. However, it is evident that such an electrode thickness is so small that the ohmic loss will increase, making it impossible to obtain a sufficient attenuation of the filter.
If the thickness of the electrode is set to around 1000 .ANG. to keep the ohmic loss sufficiently low, the electrode is too thick as compared with the thickness of the AT cut quartz crystal substrate (about 17 .mu.m), and consequently, the amount of vibration energy entrapped or confined becomes excessive and satisfactory acoustic coupling between the electrodes cannot be obtained unless the inter-electrode gap is reduced by a large amount accordingly, the passband width of the filter inevitably becomes very small. If the inter-electrode gap is reduced to such an extent as to provide desired acoustic coupling so as to avoid the above-noted problem, a highly accurate mask is needed for vapor deposition of the electrodes, besides the possibility of shorting between the divided electrodes increases, making it virtually impossible to manufacture a filter of a passband wide enough for practical use.
In addition, substantially no studies have been made so far as to what method should be used to obtain desired acoustic coupling between electrodes disposed closely to such an extent as not to cause shorting therebetween in the case of employing such a resonator as a multimode filter, partly because the resonator has been confined to studies in laboratories and has not been mass-produced.
The present invention is based on the fact that in the case of making a multimode filter element by forming split electrodes on an ultrathin AT cut quartz crystal plate as mentioned above, the passband of the filter element varies substantially with the direction of alignment of the electrodes, and it is an object of the invention to provide an ultrathin multimode quartz filter crystal element which is capable of obtaining desired acoustic coupling between electrodes in close proximity without causing shorting therebetween and thus is well suited for practical use.